Malfunctions of a steam turbine mechanical control system

Abstract This paper is aimed at elucidating the cause of a series of malfunctions involving the bending or breaking of main steam turbine throttle valve spindles which occurred at service times ranging from hundreds to several thousand hours in a number of 270 MW steam raising units. It was clearly established, by two distinct approaches (one engineering, one micromechanistic) that the stresses which produced these malfunctions were bending in nature and were the result of out-of-alignment deflections. In the case of the bent spindles the stresses were very high and approached flow strength levels of around 8000 MPa while the broken spindles were the results of fatigue initiation and subsequent growth from a thread root (stress concentration) location on the spindle. Using relevant fatigue crack propagation data for the valve spindle material at 300°C it was demonstrated that fatigue failures occurred at spindle deflections of between 0.9 and 1.6 mm. Finally, it was demonstrated that the fatigue breakage problem could be significantly reduced, especially at the lower end of the valve spindle deflection range, by a combination of re-profiling the thread root and shot peening.